1. Field of the Invention
The present invention relates, in general, to a spacer grid for nuclear fuel rods and, more particularly, to a spacer grid for dual-cooled nuclear fuel rods, capable of supporting the dual-cooled nuclear fuel rods constituting a nuclear fuel assembly used in the core of a light water reactor.
2. Description of the Related Art
A spacer grid for nuclear fuel rods is a principal component of a nuclear fuel assembly used in the core of a light water reactor, and stably positions a plurality of nuclear fuel rods, which constitute the nuclear fuel assembly, up to the end of their viable period under severe conditions in a preset space of the reactor core.
Meanwhile, a coolant flows around the nuclear fuel assembly at high speed. Thus, this coolant may cause the nuclear fuel rods to experience fluid-induced vibration. The spacer grid for nuclear fuel rods functions to inhibit such fluid-induced vibration of the nuclear fuel rods.
FIG. 1 is a schematic front view illustrating a conventional nuclear fuel assembly. FIG. 2 is a schematic top plan cross-sectional view taken along line I-I′ of FIG. 1. FIG. 3 is a schematic perspective view illustrating a spacer grid constituting the nuclear fuel assembly of FIG. 1. FIG. 4 is a schematic top plan view illustrating the spacer grid of FIG. 3.
FIG. 5 is a schematic perspective view illustrating a unit spacer grid strap of the spacer grid of FIG. 3.
Referring to FIGS. 1 through 5, the nuclear fuel assembly 10 includes fuel rods 11, an upper end fitting 12, a lower end fitting 13, guide tubes 14, and spacer grids 15. Each fuel rod 11 includes a cylindrical uranium sintered compact (called a cylindrical uranium pellet) in a zirconium alloy cladding tube. The uranium pellet causes a nuclear fission reaction which generates high-temperature heat.
Each guide tube 14 adjusts the output of a reactor core, and is used as a passage for a control rod which moves up and down in order to stop the nuclear fission reaction.
Each spacer grid 15 is usually formed of zircaloy, and includes nuclear fuel rod cells into which the nuclear fuel rods are inserted, and guide tube cells into which the guide tubes are inserted.
Each nuclear fuel rod cell of the spacer grid is configured so that a total of two spacer grid springs 28 and a total of four dimples 29 support the nuclear fuel rod 11 at a total of six supporting points, wherein the two spacer grid springs 28 are located one by one on two respective faces of the nuclear fuel rod cell, and the four dimples 29 are located in pairs on upper and lower sides of each spacer grid spring 28 on the other two faces of the nuclear fuel rod cell.
If the springs 28 and the dimples 29 are too low in elasticity, it is difficult to arrange the nuclear fuel rod 11 at a preset position, so that there is a possibility that supportability of the nuclear fuel rod 11 will become unsound.
In contrast, if the springs 28 and the dimples 29 are too high in elasticity, defects such as scratches may occur on a surface of the nuclear fuel rod 11 as a result of excessive frictional resistance occurring when the nuclear fuel rod 11 is inserted into the nuclear fuel rod cell.
Further, it is impossible to properly cover lengthwise growth of the nuclear fuel rod 11, which is caused by neutron irradiation occurring during operation of the nuclear reactor, so that the nuclear fuel rod 11 becomes bent, i.e. the nuclear fuel rod 11 can exhibit the phenomenon of flexure.
When bent, the nuclear fuel rod 11 approaches or contacts other neighboring nuclear fuel rods 11, so that the coolant channel between the nuclear fuel rods is narrowed or blocked.
Here, the coolant rapidly flows from bottom to top of the reactor core in an axial direction through sub-channels 25, each of which is surrounded by four nuclear fuel rods 11 or by three nuclear fuel rods 11 and one guide tube 14.
In other words, the sub-channel 25 refers to a space that is surrounded by the nuclear fuel rods 11, and particularly to a passage whose circumference has spaces which allow a fluid to freely move to the adjacent fluid channels.
As described above, when the coolant channel is narrowed or blocked, the heat generated from the nuclear fuel rod is not effectively transmitted to the coolant, thereby increasing a temperature of the nuclear fuel rod. As such, the possibility of generating departure from nucleate boiling (DNB) is increased, which is the main cause of a reduction of nuclear fuel output.
The upper end fitting 12 and the lower end fitting 13 function to fix and support the nuclear fuel assembly 10 to and on upper and lower structures of the reactor core. The lower end fitting 13 includes a filter for filtering foreign materials flowing in the reactor core (i.e. a foreign material filter, not shown).
FIG. 6 is a schematic top plan cross-sectional view illustrating a dual-cooled nuclear fuel rod, and FIG. 7 is a schematic top plan view illustrating a nuclear fuel assembly into which the annular nuclear fuel rods of FIG. 6 are inserted.
Referring to FIGS. 6 and 7, the dual-cooled nuclear fuel rod has an annular structure instead of a cylindrical structure, and is disclosed in U.S. Pat. Nos. 3,928,132 and 6,909,765.
The dual-cooled nuclear fuel rod 30 having the annular structure includes an annular pellet 31, an inner cladding tube 32 installed on the inner circumference of the pellet 31, and an outer cladding tube 33 installed on the outer circumference of the pellet 31.
The dual-cooled nuclear fuel rod 30 having this structure allows the coolant to flow in the outside as well as in the inside of the dual-cooled nuclear fuel rod 30, so that double heat transfer can occur so that the center of the dual-cooled nuclear fuel rod 30 can be maintained at a low temperature. In addition, the dual-cooled nuclear fuel rod 30 increases a heat transfer area to allow high burnup and output to be obtained.
As described above, when the center temperature of the dual-cooled nuclear fuel rod 30 is kept low, the possibility of the fuel being damaged as a result of the center temperature of the dual-cooled nuclear fuel rod 30 increasing is lowered, so that the safety margin of the dual-cooled nuclear fuel rod 30 can be increased.
However, in order to be structurally compatible with an existing pressurized light water reactor core, the position of each guide tube 14 cannot be changed in the nuclear fuel assembly 10, and the dual-cooled nuclear fuel rod 30 has an outer diameter greater than that of an existing cylindrical nuclear fuel rod. As such, a gap between the dual-cooled nuclear fuel rods is considerably narrowed compared to that between the cylindrical nuclear fuel rods.
For example, in the case where the nuclear fuel assembly is configured to have the dual-cooled nuclear fuel rods inserted in a 12×12 array, the gap between the dual-cooled nuclear fuel rods is reduced from 3.35 mm, which is the existing gap, to about 1.24 mm.
Thus, due the narrow gap between the dual-cooled nuclear fuel rods, the spacer grids that have been developed up to now cannot be used for the dual-cooled nuclear fuel rods 30 without being changed.
In detail, the thickness, 0.475 mm, of a unit spacer grid strap of the existing spacer grid is subtracted from the gap, 1.24 mm, between the dual-cooled nuclear fuel rods, and than the obtained result is divided into halves again. As a result, the gap between the unit spacer grid strap and the nuclear fuel rod merely amounts to about 0.383 mm.
It is impossible to design the spring having rigidity and hydraulic characteristics (mainly, pressure loss) of the existing spacer grid by applying the shape and supporting point of the existing spring within a gap which is as narrow as this. Further, the channel of the coolant is reduced by this narrow gap, so that a cooling function of the coolant is reduced.